The present invention relates to an antenna coupler for use in a RF (Radio Frequency) transmitting system.
The two major components of a transmitting station are the transmitter and the antenna. The transmitter generates a high power RF signal and the antenna receives this signal and radiates it. To insure that most of the RF power is properly coupled to the antenna, the antenna must match the output impedance of the transmitter. Unfortunately, in broadband HF systems, unless a large elaborate antenna system is employed, the antenna will not provide a good impedance match over the entire frequency range. Therefore, for simple antenna installations, a device called an antenna coupler must be used to maximize the radiated power of the station.
An antenna coupler is a variable impedance matching network designed to match the impedance of the antenna to that cf the transmitter. It is usually composed of an L-network or a T-network using servo driven, variable capacitors and roller inductors, although more modern implementations use digitally switched inductor and capacitor banks. They also include simple impedance detectors and control logic to facilitate automatic tuning.
Each time the frequency is changed, the antenna coupler must go through a tune sequence. This involves setting the network to a known home position, and then while transmitting, changing one of the elements in one direction until a desired condition takes place, as for instance, the sign of the phase changing. Then another element of the network is changed in a desired direction until its desired condition occurs, bringing the coupler to a closer match. This process is repeated until the desired impedance match is obtained and the process stops. Normally, the tune sequence takes a few seconds to complete. Unfortunately, the tune sequence of such tuners is too slow to track FM CW signals (or swept signals) or to avoid detection by direction finding equipment in a military environment.